Transient Reflection Spectroscopy Research Articles

Ultrafast charge transfer in operating bulk heterojunction solar cells.

The ultrafast charge generation process in organic solar cell devices is investigated by transient reflection spectroscopy on five state-of-the-art bulk heterojunction systems. The charge generation process in operating devices is found to be a combination of an ultrafast generation mechanism over several hundred femto-seconds and a slow process from pico-seconds to nanoseconds, limited by exciton diffusion dynamics. In addition, the lack of electric field dependence in the charge dynamics rules out geminate recombination as an important loss mechanism.

Ultrafast free-carrier dynamics in Cu2ZnSnS4 single crystals studied using femtosecond time-resolved optical spectroscopy

We studied the dynamics of photogenerated carriers in Cu2ZnSnS4 (CZTS) single crystals using femtosecond transient reflectivity (TR) and optical pump-THz probe transient absorption (THz-TA) spectroscopy. The TR and THz-TA decay dynamics consistently showed that free carriers have long lifetimes of up to a few nanoseconds. The excitation-photon-energy-dependent TR measurements revealed a slow picosecond energy relaxation of free carriers to the band edge in CZTS. The relaxation and recombination dynamics of free carriers were affected by nonradiative recombinations at the surface. Our results revealed a global feature of energy relaxation and recombination processes of free carriers in CZTS single crystals.

Photoluminescence saturation and exciton decay dynamics in transition metal dichalcogenide monolayers

We report a photoluminescence (PL) and transient reflection spectroscopy study of exciton dynamics in monolayer transition transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that the nonlinear exciton-exciton annihilation is the dominant exciton decay process in monolayer MoSe2 in contrast to the previously reported linear exciton decay in monolayer MoS2. In addition, the exciton lifetime in MoSe2, > 125 ps, is more than an order of magnitude longer than the several-ps exciton lifetime in MoS2. We find that the dramatically different exciton decay mechanism and PL saturation behavior of MoSe2 and MoS2 monolayers can be explained by the difference in their exciton lifetime.

Ultrafast transient reflectance of epitaxial semiconducting perovskite thin films

Ultrafast pump-probe transient reflectance (TR) spectroscopy was used to study carrier dynamics in an epitaxial perovskite oxide thin film of LaFeO3 (LFO) with a thickness of 40 unit cells (16 nm) grown by molecular beam epitaxy on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT). TR spectroscopy shows two negative transients in reflectance with local maxima at ∼2.5 eV and ∼3.5 eV which correspond to two optical transitions in LFO as determined by ellipsometry. The kinetics at these transients were best fit with an exponential decay model with fast (5–40 ps), medium (∼200 ps), and slow (∼ 3 ns) components that we attribute mainly to recombination of photoexcited carriers. Moreover, these reflectance transients did not completely decay within the observable time window, indicating that ∼10% of photoexcited carriers exist for at least 3 ns. This work illustrates that TR spectroscopy can be performed on thin (<20 nm) epitaxial oxide films to provide a quantitative understanding of recombination lifetimes, which are important parameters for the potential utilization of perovskite films in photovoltaic and photocatalytic applications.

Coherent nanoscale optical-phonon wave packet in graphene layers

Coherent large-wave-vector phonons in graphene layers were excited by using a 7.5-fs ultrashort laser, and observed by wavelength-resolved pump-probe transient reflectivity spectroscopy. Inter-Dirac-cone electron scattering mediated by edges and defects in graphene layers drives large-wave-vector coherent $D$-mode phonons near the Dirac point ($K$ point) of the Brillouin zone. In contrast to the normal coherent phonons generated with first-order Raman process, phonon chirp is observed, and amplitude and frequency strongly change depending on the probed wavelength. The results are discussed on the basis of doubly resonant Raman scattering process, demonstrating that generated $D$-mode coherent phonons can propagate as nanoscale optical-phonon wave packets.

Photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals

We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly localized to shallow tail states within a typical time constant of several picoseconds to a few tens of picoseconds. The sub-nanosecond PL decay dynamics indicate the importance of multiple carrier trapping processes in the shallow tail states. Therefore, it is concluded that the tail states dominate the optical responses of Cu2ZnSnS4 single crystals.

Terahertz time-domain spectroscopy of absorbing materials

In this paper, we study the noise-limited precision on the complex refractive index value of materials in the terahertz frequency range as determined by terahertz time-domain spectroscopy. As absorbing samples are almost opaque, transmission measurements are not suitable to determine the refractive index and reflection indices have to be employed instead, but at the expense of precision. We give rules to select the most adapted technique, i.e. in transmission or in reflection, for any given sample of arbitrary transparence Full Text: PDF References M. van Exter and D. Grischkowsky, "Optical and electronic properties of doped silicon from 0.1 to 2 THz", Appl. Phys. Lett. 56, 1694 (1990); CrossRef Sensing with Terahertz Radiation, ed. D. Mittleman, Springer Series in Optical Sciences Vol. 85, Berlin (2003). DirectLink For example: http://www.menlosystems.com; , http://www.rainbowphotonics.com; , http://www.teraview.com; , http://www.ekspla.com; , http://www.picometrix.com; , http://www.zomega-terahertz.com; . T. Dorney, R. Baraniuk, and D. Mittleman, "Material parameter estimation with terahertz time-domain spectroscopy", J. Opt. Soc. Am. A 18, 1562 (2001); CrossRef I. Pupeza, R. Wilk, and M. Koch, "Highly accurate optical material parameter determination with THz time-domain spectroscopy", Opt. Exp. 15, Issue 7, 4335 (2007). CrossRef L. Thrane, R. H. Jacobsen, P. U. Jepsen, and S. R. Keiding, "THz reflection spectroscopy of liquid water", Chem. Phys. Lett. 240, 330 (1995); CrossRef S. C. Howells and L. A. Schlie, "Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz", Appl. Phys. Lett. 69, 550 (1996). CrossRef P. Uhd Jepsen and B. M. Fischer, "Dynamic range in terahertz time-domain transmission and reflection spectroscopy", Optics Letters 30, 29 (2005). CrossRef L. Duvillaret, F. Garet, J.-L. Coutaz, "Influence of noise on the characterization of materials by terahertz time-domain spectroscopy", J. Opt. Soc. Amer. B17, 452 (2000). CrossRef

Preface

The unique success story of semiconductor physics and technology relies on the ability to highly integrate micro- and nanometer sized functional units on a single chip. Within the last few years single nanostructures, e.g., individual and coupled pairs of quantum dots and single molecules moved more and more into the focus of many research groups to study the exciting physics and to exploit their tremendous potential for device applications. The full advantage of their superior properties can be utilized if a controlled positioning or growth of the nanostructure inside a more complex device structure can be realized, thereby defining precise coupling between the microscopic nanostructure and the macroscopic periphery. single-photon sources coupled quantum-dot gates single quantum-dot tunneling diodes. electroluminescence of single electrically-driven molecules at room temperature controlling quantum-dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators controlled coupling of individual self-assembled quantum dots to plasmonic nanoantennas quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field realization of vertically stacked and laterally ordered InP and InAs quantum dots for quantum gate applications development of site-controlled SiGe islands on patterned Si(001) and realization of tunneling diodes ultrafast transient reflection spectroscopy of a single self-assembled GaAs quantum dot. We hope that this collection of ideas and results of controlled positioning of single nanostructures stimulates many fascinating experiments and clever applications of quantum objects. We wish to thank all people involved in the research group for their excellent work, the DFG for the financial support and Wiley for the opportunity to publish this volume. On behalf of all applicants being funded by DFG over the years, M. Lippitz and P. Michler Stuttgart, March 2012

Transient absorption spectroscopy of a single lateral InGaAs quantum dot molecule

AbstractLateral quantum dot molecules promise a scalable approach to quantum registers. The individual neutral exciton states can be controlled by a lateral bias voltage. In this way radiative emission of localized excitons can be switched between the two quantum dots in the molecule. Here, we report on coherent absorption spectroscopy of a single quantum dot molecule. By transient reflection spectroscopy we demonstrate that not only the emission but also the absorption switches from one dot to the other when varying the applied electric field. This result is consistent with Rabi flopping showing a radically suppressed dipole moment for the non‐emitting exciton transition

Ultrafast coherent spectroscopy of a single self‐assembled quantum dot

AbstractThe controlled interaction of several quantum dots (QDs) mediated by plasmonic or photonic nanostructures promises interesting new functionality in the fields of quantum computing and telecommunication. Ultrafast laser pulses can be used to write and read out the state of the QD. We review ultrafast coherent spectroscopy of single QDs. The focus of this article is on the technique of transient reflection spectroscopy which can be applied to a broad range of samples and devices. It only requires optical access to a single quantum system next to a reflecting surface. We demonstrate the versatility of our approach by presenting several quantum optical studies such as Rabi oscillations, perturbed free induction decay, and quantum beats from an entangled excitonic state in weakly absorbing GaAs QDs. We expect this experimental method to make coherent experiments possible in elaborate devices where quantum emitters are interacting with a complex environment such as plasmonic waveguides or antennas.

Ultrafast dynamics of photoinduced phenomena in the spin ladder system NaV2O5

Recent developments in the study of ultrafast dynamics are reviewed concerning the spin ladder system, α'-NaV2O5, which is known to have a charge order phase transition accompanying spin dimerization at 34 K. The low energy excitations specific to the low temperature phase is observed by coherent excitation method and their origins are discussed, comparing with the Raman and infrared absorption data. Photoinduced phenomena are studied by transient reflectance and time-resolved Raman spectroscopy and the thermal and nonthermal effects are distinguished by their response time.

Nonlinear Optical Dynamics During Phase Transition in Vanadium Dioxide

ABSTRACTInsulator-to-metal phase transition (PT) and relaxation dynamics in VO2 were studied with ultrafast transient reflection spectroscopy and degenerate four wave mixing technique. Optical pumping of free carriers in VO2 initiates PT on a femtosecond time scale. The relaxation dynamics is strongly dependent on pump energy. Spectral reflectivity behaviour at PT due to thermal heating and upon laser excitation of VO2 demonstrates close proximity in the relative change. PT induced by light was interpreted by exciton-controlled mechanism. Excited state dynamics of metallic VO2 is assigned to formations of electronic and hole polaronic states.

Time-frequency spectroscopy of an InGaAs/InP quantum-well exciton Bragg reflector

Ultrafast absorption saturation in an exciton Bragg reflector, consisting of InGaAs quantum wells embedded in InP/InGaAsP Bragg layers, is characterized by pump-induced transient reflectance spectroscopy at room temperature. Spectral modulation of the reflected probe pulses is caused by ultrafast absorption saturation of excitons in the quantum wells. The spectral modulation leads to an optical phase shift of 1.6 π. The exciton-induced phase shift is more than four times as large as that induced by free carriers in a Bragg reflector.

Photochemistry of acenaphthene at a silica gel/air interface

The photolysis of acenapthene (ACE) has been studied at a silica gel/air interface. Direct photolysis leads to 1-acenaphthenol as the principal photoproduct. Secondary photochemical conversion of the alcohol into 1-acenaphthenone is also observed. Reactions leading to formation of both 1-acenaphthenol and 1-acenaphthenone are attributed to electron-transfer oxidation mechanisms and the cation radical of ACE is directly observed by transient diffuse reflectance spectroscopy. Preconditioning the silica gel by heating at 200°C in air partially removes physisorbed water from the silica surface and photochemical conversion of ACE is observed to occur faster.

Coherent reflected pulses of exciton polaritons in multiple quantum wells at Brewster’s-angle incidence

A coherent emission of excitons in semiconductor multiple quantum wells was investigated by using transient Brewster reflection spectroscopy. The method reduced nonresonant background reflection greatly and allowed us to observe the coherent dynamics of the excitons clearly. The coherent signal showed not only the quantum beats of the heavy- and light-hole excitons but also the oscillatory modulation structures that resulted from the radiative coupling of exciton polaritons. In the linear regime, these observations were compared with results calculated from the transfer-matrix method involving multiple-reflection effects, and good agreement was obtained. As the excitation density of the incident pulses was raised, the decay rate of the coherent signal increased due to the exciton-exciton scattering.

Spectroscopy and photochemistry of fluorene at a silica gel/air interface

The emission spectroscopy of fluorene has been studied at a silica gel/air interface at surface loadings from 1.5% to 77.1% of a monolayer. Both monomer and excimer-like emissions are observed on the surface. The excimer-like emission, centered at 350 nm, arises from excitation of ground-state van der Waals pairs of fluorene molecules. Photolysis into the van der Waals dimers at room temperature does not lead to dissociation into constituent monomers; however, irradiation of fluorene monomers at a silica gel/air interface leads to production of 9-fluorenone as the only identifiable photoproduct in 72% yield. Fluorene triplet and cation radical are both observed at the silica gel/air interface by transient diffuse reflectance spectroscopy. Sensitization of singlet molecular oxygen by silica-sorbed methylene blue, in the presence of co-sorbed fluorene, does not lead to fluorene oxidation, thus suggesting that oxidation by direct photolysis does not involve an energy transfer mechanism. An electron transfer oxidation mechanism is proposed to account for the observed photochemical oxidation of fluorene at the silica gel/air interface.

Visible Light Photolysis of Hydrogen Iodide Using Sensitized Layered Metal Oxide Semiconductors: The Role of Surface Chemical Modification in Controlling Back Electron Transfer Reactions

The internally platinized wide bandgap semiconductor K4Nb6O17 can be sensitized by [(bpy)2Ru(4-(2,2‘-bipyrid-4-yl)-phenylphosphonic acid](PF6)2 (1). In aqueous iodide solutions at pH 2, the visible light photolysis of HI, to form H2 and I3-, is catalyzed by 1/K4-xHxNb6O17/Pt. The strong bond between the surface and the phosphonate group of 1 allows one to adsorb other surface species, which decrease the rate of the back electron transfer reaction between conduction band electrons and I3- ions. Methylphosphonic acid and undecylphosphonic acid do not form good surface monolayers on 1/K4-xHxNb6O17 and do not increase the rate of hydrogen evolution. Anionic surface modifiers [TiNbO5]nn-, derived from exfoliation of KTiNbO5, and poly(styrenesulfonate), PSS, increase the initial hydrogen evolution rate by factors of 3 and 5, respectively. In the latter case, the initial quantum yield for HI photolysis is ca. 3%. Transient diffuse reflectance spectroscopy was used to monitor the formation and disappearance of I3- ions with 1/K4-xHxNb6O17 and PSS/1/K4-xHxNb6O17. The rate constant for the back electron transfer reaction between conduction band electrons and I3- ions decreases from 3.17(±0.03) × 107 to 3.01(±0.02) × 106 M-1 s-1 upon adsorption of PSS.

Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz

Using transient terahertz reflection spectroscopy, the far-infrared (0.1–1.1 THz) reflection amplitude and associated phase change upon reflection from undoped InSb was measured between 80 and 260 K. Using the Drude model, the transient terahertz data were fit by adjusting the values for the electron mobility and carrier concentration. The close agreement between the data and model validates using the Drude model to describe the electronic and optical properties of InSb below 1 THz. These results suggest a possible application of transient terahertz reflection spectroscopy for measuring semiconductor properties of thick samples without surface contact.